Sheet-metal formed component and method for the production of the sheet-metal formed component

ABSTRACT

The invention relates to a sheet-metal formed component and a method or producing the sheet-metal formed component, produced by hot-working and press-quenching from a quenchable, unitary and materially uniform steel alloy, wherein the sheet-metal formed component has multiple superposed martensite layers, wherein a respectively outer martensite layer of the sheet-metal formed component has higher ductility than an underlying martensite layer.

RELATED APPLICATIONS

The present application is a National Phase of International Application Number PCT/DE2018/100530 filed Jun. 1, 2018, which claims priority to German Application Number 10 2017 112 164.1 filed Jun. 1, 2017.

FIELD

The present disclosure relates to a sheet-metal formed component, produced by hot-working and press-quenching.

The present disclosure further relates to a method for the production of the sheet-metal formed component, and to a method for the production of a metal semi-finished product.

BACKGROUND

The production of sheet-metal formed components is known from the prior art. To that end, sheet-metal blanks are shaped using conventional shaping methods, for example deep drawing, to give a three-dimensionally shaped component. Sheet-metal formed components of this kind are used to a great extent in the motor vehicle industry and in that context are used as motor vehicle components. Consequently, and in the context of this disclosure, sheet-metal formed components are to be understood essentially as motor vehicle components.

In the context of motor vehicles, a distinction is drawn between, among other things, motor vehicle structural components which are used to produce a self-supporting motor vehicle body. Known examples of these are motor vehicle pillars, an A-pillar or a B-pillar, longitudinal beams, transverse beams, roof spars, door sills, transmission tunnels or similar components. Body outer skin components of the motor vehicle can also be produced, for example an engine hood, a roof outer skin or a door outer skin. It is also possible to produce add-on parts or crash components, for example a crash box, a bumper crossmember or the like.

However, the motor vehicle industry requires consistent implementation of lightweight construction methods as well as improved stiffness or, as the case may be, crash properties of the components. Hot-working and press-quenching technology was developed for this purpose. This makes it possible to heat a blank or a preformed semi-finished product made of a quenchable steel alloy to a temperature above the austenizing temperature (AC3). The blank is shaped in this hot state. This has the advantage, on one hand, that the heating to above the austenizing temperature increases the degree of shaping of the blank that it is possible to generate. Already during and/or after shaping, the still-hot sheet-metal formed component is cooled at such a high rate that the structure changes from austenite to martensite, making it possible to set high strengths.

The components in this manner by hot-working and press-quenched have high strength. However, the high strength can also be accompanied by brittleness or reduced ductility of the sheet-metal formed component produced in this manner.

However, this is generally not desired since it can lead to brittle fractures and, in the event of a crash, to the sheet-metal formed component tearing off at coupling points.

Use is generally made of conventional furnace heating systems which have for example a 30 to 40 m-long heating path. Accordingly, an associated heating time to above the austenizing temperature is required.

The sheet-metal formed components produced by hot-working and press-quenching, for example made of a steel of the 22MnB5 type, have good properties with regard to strength and at the same time ductility.

However, in recent years contact heating has become known, specifically in the field of hot-working and the heating to above the austenizing temperature required for that purpose. In this context it is possible, with a small footprint in a production hall with at the same time high heating rates of greater than 30 K/s, or greater than 50 K/s, to heat the blanks much more rapidly for hot-working and subsequent press-quenching. However, it has been observed that, when using known quenchable steel alloys, although high strengths are achieved in the finished product, the short heating time leads to reduced ductility and consequently reduced bending angle. This makes the production of crash-relevant components impossible or unreliable with rapid heating.

SUMMARY

The present disclosure therefore has the object, proceeding from the prior art, of specifying a component and a method for the production of the component that overcome the above-mentioned drawbacks.

The above-mentioned object is achieved, according to the disclosure, with a sheet-metal formed component, produced by hot-working and press-quenching.

The method-relevant part of the object is further achieved by a method for the production of the sheet-metal formed component.

A further method-relevant part of the object is achieved by a method for the production of a metal semi-finished product.

The sheet-metal formed component according to the disclosure is produced by hot-working and press-quenching. The sheet-metal formed component is in that context produced from a quenchable, unitary and materially uniform steel alloy. This means that it is not a plated material but rather a material that is unitary and materially uniform in section. In that context, the sheet-metal formed component has a tensile strength Rm of greater than 1200 MPa, or greater than 1350 MPa. Moreover, the sheet-metal formed component has a bending angle of greater than 60° for a wall thickness of 0.5 to 1.5 mm. For a greater wall thickness of 1.5 to 2.5 mm, the sheet-metal formed component has a bending angle of greater than 45°. The tensile strength should not exceed 2500 MPa.

The bending angle is determined in the plate bending test according to VDA 238-100:2010, at a proof stress Rp 0.2 of greater than 900 MPa.

According to the disclosure, the sheet-metal formed component is henceforth characterized in that, proceeding from both surfaces, in each case laminated martensite plies or martensite layers are formed. Consequently, from an upper side and an underside of the three-dimensionally shaped sheet-metal formed component, adjacent martensite layers with different properties are formed over the sheet thickness, or wall thickness. These are, in alternation, more ductile and harder martensite layers. The more ductile martensite layer is always on the surface, or outer side.

According to the disclosure, the above-mentioned sheet-metal formed component is produced from a hot-rolled product, hereinafter also termed semi-finished product. The hot-rolled product is produced so as to be unitary and materially uniform. However, at the end of the rolling process it does have different layers in the material structure. The layers can also be termed plies or lines. The layers are areal and extend over the entire surface area of the resulting semi-finished product, but at least over the entire strip breadth. The semi-finished product is provided in the form of a blank.

According to the disclosure, the respective outer layer of the semi-finished product is designed as a ferrite layer. This in turn has a thickness of 4 to 140 μm. Consequently, an outer ferrite layer is created on the upper side and the underside of the semi-finished product. Beneath this ferrite layer, a pearlite layer is created with a thickness of 4 to 25 μm. Adjoining this are, respectively in alternation, further ferrite and pearlite layers over the strip thickness or wall thickness. The layers always extend over the entire strip breadth.

From the semi-finished product that is laminated in this manner, following the rapid heating, hot-working and subsequent cooling during press-quenching, an outer carbon-poor martensite layer and an underlying carbon-rich martensite layer can be created, since the rapid heating causes no diffusion equalization between the ferritic and pearlitic layers. Thus, the outer ferrite layer is converted to a martensite layer which has lower strength while at the same time having high ductility. The underlying pearlite layer is converted to a martensite layer which has, by contrast, higher strength but reduced ductility.

Over the wall thickness, there are at least three, five or seven layers of martensite formed.

A delta or difference in the strength between a martensite layer having higher strength but reduced ductility and the martensite layer having lower strength but higher ductility is at least between 100 and 300 MPa. This means that the higher-strength martensite layer is at least 100 to 300 MPa stronger than the martensite layer having greater ductility but lower strength. However, the delta between the individual martensite layers of different strength should not exceed approximately 1000 MPa.

It can further be provided, by additional targeted surface decarburization of the semi-finished product, to create the outermost layer as a surf ace-decarburized layer which has a very low carbon content.

In this surf ace-decarburized layer, the very carbon-poor ferritic material structure that is present does not convert to martensite during press-quenching, or does so only to a limited degree, and as a result this structure has a markedly lower strength. The surf ace-decarburized layer has an essentially ferritic material structure. In comparison to the higher-strength and reduced-ductility martensite layer, the difference in strength can be up to 1000 MPa.

These layers having different strengths and different ductilities then alternate over the strip thickness. However, the overall hot-worked and press-quenched sheet-metal formed component has high strength and at the same time high ductility so that the above-mentioned bending angle can be achieved even in spite of the rapid heating of the semi-finished product.

Surface decarburization can be carried out simultaneously or additionally. Thus, in the produced sheet-metal formed component, the ductility in the surface region is further increased while the strength remains constant. Surface decarburization is carried out after hot-rolling of the sheet-metal strip.

The respectively outer layer of the sheet-metal formed component, that is to say the outer martensite layer, has a layer thickness of 4 to 140 μm, 10 to 140 μm, or 14 to 140 μm.

If the sheet-metal formed component has an optional surface decarburization, this surface decarburization is external, extending in each case from the surface into the sheet-metal formed component, and is thus included in the above-mentioned layer thickness of the outer layer, or the surf ace-decarburized layer can also form the outer layer. For example, a surface decarburization can extend in a layer from 10 to 140 μm, or 20 to 100 μm from the surface into the sheet-metal formed component, or into the outer martensite layer.

For the production of the sheet-metal formed component according to the disclosure, use can be made of quenchable steel alloys such as 22MnB5, but also MBW 1900 or MBW 1500. These are manganese boron steels have the following modification: it has been found to be advantageous, for the production of the semi-finished product, to use a quenchable manganese boron steel having a proportion by weight of 0.5 to 1.7% (inclusive) of manganese (Mn) and a proportion of 0.0008 to 0.005% (inclusive) of boron (B). The manganese content slows the incubation time for the formation of bainite and ferrite. The boron content slows the formation of ferrite and pearlite. This combination of the alloying elements makes it possible to generate an isolated ferritic-pearlitic conversion region starting at the surface so that, through the cooling conditions imposed after rolling, it is possible to create a lattice structure/lattice layers in a targeted manner in the semi-finished product. The targeted cooling in a cooling path after the final rolling stand makes it possible, in that context, to establish a laminated structure of ferrite and pearlite over the strip thickness, proceeding from the surface. The subsequent rapid heating, hot-working and press-quenching makes this convert into corresponding martensite layers with mutually different strength properties. A fine martensite structure with locally different carbon contents is created in the ferrite layer and the pearlite layer.

The produced sheet-metal formed components are motor vehicle components, such as body components, or body structural components which meet crash-relevant requirements.

The rapid heating in less than 1 minute, 30 s, or 20 s, with a heating rate of greater than 30 K/s, or greater than 50 K/s, from room temperature to above the AC3 temperature, is carried out in the context of the disclosure by contact heating. To that end, contact plates are applied to one side or to both sides of the semi-finished product, that is to say the blank. The contact plates are at a higher temperature so that through thermal conduction the higher temperature of the contact plates is transferred to the semi-finished product that is to be heated. Inductive heating, and heating by means of a burner flame or infrared, are also possible.

The use of contact heating technology makes it possible, in a quasi-targeted manner, to temper only partial regions. This makes it possible to heat only partial area regions of the semi-finished product to above the austenizing temperature, which then leads, in the subsequent hot-working and press-quenching, to only partial hardening in these area regions. In the partial area regions, the tempering and hardening takes place over the entire wall thickness. In the case of the hardening, the various martensite layers are created.

It is also possible to produce a semi-finished product that has different wall thicknesses in certain area regions.

The present disclosure further relates to a method for the production of a sheet-metal formed component having the following method steps:

-   -   hot-rolling a quenchable steel alloy,     -   generating a ferrite-pearlite lamination with, on the surface of         the hot-rolled product, a ferrite layer over the entire strip         breadth,     -   individualizing to give blanks,     -   rapid heating of a blank, in a time of less than 60 s and at a         heating rate of greater than 30 K/s, 50 K/s, 60 K/s, or 80 K/s,         from ambient temperature to above the austenizing temperature,     -   hot-working and press-quenching of the sheet-metal formed         component.

In the above-mentioned method, the metal semi-finished product used is a hot-rolled product which is produced by means of the method described below:

-   -   hot-rolling a quenchable steel alloy,     -   generating an outer ferrite-pearlite lamination with, on the         surface of the hot-rolled product, a ferrite layer over the         entire strip breadth,     -   individualizing to give blanks.

The hot-rolling can be carried out with the following method parameters:

-   -   providing a slab and heating to a core temperature of above         1200° C. for a time of greater than 60 s,     -   rolling to a preliminary strip thickness of between 45 and 55         mm,     -   intermediate-rolling to a strip thickness of 13 to 25 mm,     -   rolling end temperature of the rolled steel sheet strip 860 to         920° C.,     -   rolling end velocity 3-12 m/s,     -   cooling over a path of 65 to 80 m after the final rolling stand         at 15 to 30 K/s,     -   reaching a reel temperature of 650 to 800° C.,     -   coiling the steel strip produced in this manner.

The steel sheet strip hot-rolled and cooled in this manner has in outer layers the ferrite and pearlite structure according to the disclosure, which, in a subsequent hot-working and press-quenching method, serves for improved ductility of the produced sheet-metal formed components in comparison to a conventionally hot-worked and press-quenched steel with preceding rapid heating.

It is also possible for the steel sheet strip to be coated, for example with an AlSi or zinc coating.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features, properties and aspects of the present disclosure form the subject matter of the following description. Configuration embodiments are described in schematic figures. Said figures serve for simpler understanding of the disclosure. In the figures:

FIG. 1 shows a manufacturing sequence for the production, first, of a semi-finished product according to the disclosure and for the further processing to give a sheet-metal formed component produced according to the disclosure,

FIG. 2 shows a partial section view through a semi-finished product according to the disclosure, and

FIGS. 3a to 3c show in each case a partial section view through a sheet-metal formed component produced according to the disclosure,

FIG. 4 shows a sheet-metal formed component produced according to the disclosure, having in certain regions areas of mutually different strengths, and

FIG. 5 shows a partial section view through a sheet-metal formed component, according to the disclosure, with different wall thicknesses.

In the figures, the same reference signs are used for identical or similar components, even if a repeated description is omitted for reasons of simplicity.

DETAILED DESCRIPTION

FIG. 1 shows a production method according to the disclosure. First, a slab 2 made of a quenchable steel alloy is provided, is heated in a furnace 14 and is then fed through a rolling path 3. The furnace 14 is at a temperature T₁ above 1200° C. After passing through the final rolling stand 4, the steel sheet strip 5 rolled in this fashion has a rolling end temperature. It is then fed through a cooling path 6. At the end of the cooling path 6, the cooled steel sheet strip 7 is at a reel temperature in order to be then coiled on a coiler 8. Thus, a hot-rolled product is provided in the form of a coil. The strip breadth B extends into the plane of the image.

The hot-rolled product can however also, according to the next method step, be an appropriately individualized blank 9. In an uncoiling process (not shown in greater detail), the steel sheet strip 7 is supplied to an individualizer 10. The individual blanks 9 then undergo, according to the disclosure, rapid heating in a tempering station 11 and are heated to above the austenizing temperature. To that end, contact plates 12, which come into contact with the blank 9 that is to be heated, are arranged in the tempering station 11.

The heated blank 9 is transferred to a hot-working and press-quenching tool 13, where it is hot-worked and press-quenched. The sheet-metal formed component 1 produced according to the disclosure is obtained at the end of the press-quenching procedure.

FIG. 2 shows a cross section of a detail of the semi-finished product, or of the individualized blank 9, prior to heating, that is to say prior to austenizing. The blank 9 has an overall wall thickness, hereinafter termed wall thickness W, which is between 0.5 and 2.5 mm. From a respective outer surface 15, 16, multiple layers of ferrite and pearlite are arranged one atop the other in alternation over the wall thickness W. The ferrite and pearlite layers are then arranged immediately next to one another. Over the wall thickness W, the blank 9, or the semi-finished product, is unitary and materially uniform.

The respective outer ferrite layer 17 has a thickness D17 of 4 μm to 140 μm. The outer ferrite layer 17 then respectively also forms the surface 15, 16 of the blank 9. Proceeding from the surface 15, or 16, a pearlite layer 18 is in each case arranged beneath the ferrite layer 17. The pearlite layer 18 has a thickness D18 of 4 μm to 25 μm. There then follow, in alternation, further ferrite layers 19, in turn followed by a respective pearlite layer 20. These can also respectively have a thickness of 4 μm to 25 μm.

In the depiction here, thirteen layers are formed over the wall thickness W. According to the disclosure, at least three layers, five layers, or more than seven layers of ferrite and pearlite are formed over the wall thickness W. The individual layers are not shown to scale with one another with regard to their respective wall thickness ratio.

FIG. 3a shows the detail of FIG. 2 from the already-produced sheet-metal formed component 1. This means that heating, hot-working and press-quenching have taken place. Furthermore, the individual layers are formed over the wall thickness W. However, the structure has transformed into martensite. What were the ferrite layers have transformed into martensite layers 21 of lower strength and high ductility in comparison to the martensite layer 22 described below. The pearlite layers 18 which lie beneath, as seen from the surface 15, 16, and also the deeper pearlite layers 20, have transformed into martensite layers 22 of higher strength and lower ductility. Deeper still, lower-strength and higher-ductility martensite layers 21 alternate with higher-strength and lower-ductility martensite layers 22.

FIG. 3b shows a detail similar to FIG. 3a , wherein here each outer surface has a layer 26 formed by surface decarburization. This has an essentially ferritic material structure, or the layers 26 can also consist entirely of ferrite. The surface-decarburized layer 26 then transitions into the outer martensite layer 21 that has low strength but higher ductility. Optionally, the surface-decarburized layer 26 can also form the entire outer layer. The higher-strength martensite layer 22 then follows directly. This is shown in FIG. 3 c.

FIG. 4 shows a sheet-metal formed component 1, produced according to the disclosure, as a motor vehicle component and in this case specifically as a motor vehicle pillar. This sheet-metal formed component 1 has, for example, a lower foot region 23, an upper roof-attachment region 24, and between these two a middle portion 25. The middle portion 25 can have a reduced wall thickness W25 compared to, for example, the foot region 23.

A partial longitudinal section view along section line A-A is shown in FIG. 5. That figure shows that the wall thickness W23 in the foot region 23 is less than the wall thickness W25 in the middle portion 25. The individual martensite layers are also formed in the region of lesser wall thickness. The individual layers are for example produced by flexible cold-rolling of the blank 9 or of the cooled steel sheet strip 7. This swages the ferrite and pearlite layers so that they are thinner but still present in the same number over the wall thickness W. Once austenizing, hot-working and press-quenching are complete, the individual martensite layers are formed also in the lesser wall thickness, in the same number but thinner.

The foregoing description of some embodiments of the disclosure has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings. The specifically described embodiments explain the principles and practical applications to enable one ordinarily skilled in the art to utilize various embodiments and with various modifications as are suited to the particular use contemplated. It should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the disclosure. 

1-10. (canceled)
 11. A sheet-metal formed component, produced by hot-working and press-quenching from a quenchable, unitary and materially uniform steel alloy, the sheet-metal formed component comprising: a plurality of superposed martensite layers, wherein one of the plurality of superposed martensite layers is an outer martensite layer and another one of the plurality of superposed martensite layers is an underlying martensite layer underlying the outer martensite layer, the outer martensite layer having higher ductility than the underlying martensite layer, the sheet-metal formed component has a tensile strength of greater than 1200 MPa, and the sheet-metal formed component has a bending angle of greater than 60° for a wall thickness of 0.5 to 1.5 mm or a bending angle of greater than 45° for a wall thickness of 1.5 to 2.5 mm.
 12. The sheet-metal formed component according to claim 11, wherein the plurality of superposed martensite layers comprises at least three martensite layers between two opposite surfaces of the sheet-metal formed component over the wall thickness, and the outer martensite layer has a thickness of 4 μm to 140 μm.
 13. The sheet-metal formed component according to claim 11, wherein the plurality of superposed martensite layers is superposed between two opposite surfaces of the sheet-metal formed component, and both of the surfaces are surface-decarburized layers comprising an ferritic material structure.
 14. The sheet-metal formed component according to claim 11, wherein the plurality of superposed martensite layers is produced from a semi-finished product having an outer ferrite layer with an underlying pearlite layer.
 15. The sheet-metal formed component according to claim 11, wherein the quenchable steel alloy comprises a manganese boron steel having a proportion by weight of 0.5 to 1.7% (inclusive) of manganese and 0.0008 to 0.005% (inclusive) of boron.
 16. The sheet-metal formed component according to claim 11, wherein certain area regions of the sheet-metal formed component have at least one of different strengths, or different wall thicknesses.
 17. A method of producing a sheet-metal formed component, the method comprising: hot-rolling a quenchable steel alloy to obtain a hot-rolled product, generating, on a surface of the hot-rolled product, a ferrite-pearlite lamination with a ferrite layer over an entire strip breadth of the product, individualizing the product to obtain blanks, rapid heating of the blanks at a heating rate of greater than 30 K/s from ambient temperature to above an austenizing temperature, and hot-working and press-quenching each of the rapid heated blanks into a sheet-metal formed component, wherein the sheet-metal formed component comprises a plurality of superposed martensite layers, one of the plurality of superposed martensite layers is an outer martensite layer and another one of the plurality of superposed martensite layers is an underlying martensite layer underlying the outer martensite layer, the outer martensite layer having higher ductility than the underlying martensite layer, the sheet-metal formed component has a tensile strength of greater than 1200 MPa, and the sheet-metal formed component has a bending angle of greater than 60° for a wall thickness of 0.5 to 1.5 mm or a bending angle of greater than 45° for a wall thickness of 1.5 to 2.5 mm.
 18. A method of producing a metal semi-finished product from a quenchable steel alloy for further processing into a sheet-metal formed component, the method comprising: hot-rolling the quenchable steel alloy to obtain a hot-rolled product, generating an outer ferrite layer and an underlying pearlite layer on both of opposite surfaces of the hot-rolled product, over an entire strip breadth of the product, and individualizing the product to obtain blanks each of which is a metal semi-finished product to be further processed into a sheet-metal formed component, wherein the sheet-metal formed component comprises a plurality of superposed martensite layers, one of the plurality of superposed martensite layers is an outer martensite layer and another one of the plurality of superposed martensite layers is an underlying martensite layer underlying the outer martensite layer, the outer martensite layer having higher ductility than the underlying martensite layer, the sheet-metal formed component has a tensile strength of greater than 1200 MPa, and the sheet-metal formed component has a bending angle of greater than 60° for a wall thickness of 0.5 to 1.5 mm or a bending angle of greater than 45° for a wall thickness of 1.5 to 2.5 mm.
 19. The method according to claim 17, wherein the hot-rolling comprises: heating a slab of the quenchable steel alloy to a core temperature of above 1200° C. for a time of greater than 60 s, rolling the heated slab first to a preliminary strip thickness of between 45 and 55 mm, and then to a strip thickness of 13 to 25 mm to obtain a rolled steel strip, wherein a rolling end temperature of the rolled steel strip is 860 to 920° C., and a rolling end velocity is 3-12 m/s, and after the rolling, cooling the rolled steel strip over a path of 65 to 80 m at a cooling rate of 15 to 30 K/s until reaching a reel temperature of 650 to 800° C. to obtain the hot-rolled product.
 20. The method according to claim 17, wherein the rapid heating is carried out by contact heating at a heating rate of greater than 50 K/s.
 21. The sheet-metal formed component of claim 11, wherein the sheet-metal formed component has a tensile strength of greater than 1350 MPa.
 22. The sheet-metal formed component according to claim 11, wherein the plurality of superposed martensite layers comprises at least five martensite layers between two opposite surfaces of the sheet-metal formed component over the wall thickness, and the outer martensite layer has a thickness of 4 μm to 140 μm.
 23. The sheet-metal formed component according to claim 11, wherein the plurality of superposed martensite layers comprises at least seven martensite layers between two opposite surfaces of the sheet-metal formed component over the wall thickness, and the outer martensite layer has a thickness of 4 μm to 140 μm.
 24. The sheet-metal formed component according to claim 11, wherein the plurality of superposed martensite layers comprises two outermost martensite layers at two opposite surfaces of the sheet-metal formed component, respectively, and the two outermost martensite layers are surface-decarburized layers having an essentially ferritic material structure.
 25. The sheet-metal formed component according to claim 14, wherein the semi-finished product has further underlying alternating ferrite layers and pearlite layers.
 26. The method according to claim 17, wherein the rapid heating is carried out by contact heating at a heating rate of greater than 80 K/s.
 27. The method according to claim 17, wherein the rapid heating of the blanks is carried out at a heating rate of greater than 50 K/s, from ambient temperature to above the austenizing temperature. 